Needleless hypodermic injection devices have been known and used in the past. These devices typically use spring or compressed gas driven plungers to accelerate a fluid at a velocity sufficient to pierce the skin and enter the underlying tissues.
Since at least the 1980s, the use of needleless injectors has become more desirable due to concerns over the spread of AIDS, hepatitis and other viral diseases caused by the possibility of accidental needle "sticks" from the conventional syringe and needle. Needleless injectors remove apprehensions of healthcare workers and are superior in eliminating accidental disease transmission.
A number of different needleless injectors are known including U.S. Pat. No. 5,062,830 to Dunlap, U.S. Pat. No. 4,790,824 to Morrow et al., U.S. Pat. No. 4,623,332 to Lindmayer et al., U.S. Pat. No. 4,421,508 to Cohen, U.S. Pat. No. 4,089,334 to Schwebel et al., U.S. Pat. No. 3,688,765 to Gasaway, U.S. Pat. No. 3,115,133 to Morando, U.S. Pat. No. 2,816,543 to Venditty, et al., and U.S. Pat. No. 2,754,818 to Scherer. These injectors typically include a nozzle assembly, which includes a medication holding chamber and a piston. The chamber has an orifice through which a jet of medication is forced out of the chamber using the piston actuated by some type of energy source.
Due to the high velocity of the jet created by the typical needleless injector, it is desirable to prevent premature discharges of the energy source. Thus, there is a need for a needleless injector which remains in the unarmed or locked configuration until the time period when activation is desired.